Along with the ongoing push to increase the density of electronic and integrated circuits, there has developed a need for higher density power converters that supply power to these circuits. That is, there has developed a need to either reduce the physical size of a power converter and keep its power output the same or keep the physical size of the power converter the same and increase its power output or both (i.e., reduce the physical size and increase the power output). By accomplishing one of the above, the power density of the power converter, i.e., electrical power output (watts) per cubic inch of volume taken up by the power converter, increases. Most power converters presently have power densities in the range of 1-3 watts/cubic inch. Next generation power converters, which are presently under development, may have densities as much as an order of magnitude larger than present power converter densities (e.g., 30 watts/cubic inch). Long term projections, however, indicate that future power converters will be required to have much higher power densities, perhaps as high as several hundred watts/cubic inch.
In order to increase the power density of present power converters to high enough levels to satisfy future needs, present power converters must be made smaller and more efficient. One way to reduce the size of a power converter is to reduce the size and/or number of components in the power converter. Similarly, the efficiency of a power converter can be increased by increasing the efficiency of the components and/or by reducing the number of components in the power converter.
As is readily apparent to a person skilled in the power conversion art, power converters typically include one or more magnetic components. An output series inductor is typically used as an energy storage device to filter the power converter output. In addition, the power converter may be coupled to a voltage supply by a transformer. The transformer typically performs two functions: transforming the magnitude of the supply voltage to a level suitable for the circuits fed by the power converter; and, isolation of those circuits from the voltage supply.
An example of a prior art power converter having these magnetic components is illustrated in FIG. 1. FIG. 1 is a simplified schematic diagram of a transformer-coupled push-pull converter 10. Push-pull converters of the sort illustrated in FIG. 1 are well known in the art and, hence, are not discussed in detail herein. However, to better understand the present invention, a few key components of the push-pull converter 10 illustrated in FIG. 1 and their function are briefly discussed. The push-pull converter 10 is coupled to an AC voltage supply 22 by a transformer 12 and comprises a rectifier 14 and an output filter 16. The rectifier 14 consists of two diodes 17 and 18 connected in a conventional manner to provide a rectified output voltage. The output filter 16 consists of an output inductor 19 and a capacitor 20. A rectified and filtered output voltage is produced by the push-pull converter 10 and applied to a load 24. Thus, as can be seen from FIG. 1, and from the above discussion, a conventional, prior art push-pull converter 10 comprises at least two magnetic devices (viz., the transformer 12 and the output inductor 19).
One problem associated with power converters in the prior art is that the transformers and inductors usually have a conventional, wound-type of construction, which makes them relatively bulky. As a result, their size limits efforts to reduce the size of the associated power converters. Yet another problem associated with conventional transformers and inductors used in prior art power converters is that their construction is very labor intensive. As a result, they are expensive to construct and not well suited to high volume production methods, which increases the cost of prior art power converters.
An important characteristic of power converters, such as the prior art pushpull converter 10, illustrated in FIG. 1, is that they be controllable. For example, in many instances the output of the power converter must be a regulated, i.e., controlled, output. Typically, the output voltage of a power converter is controlled by controlling the average value of the input voltage applied to the power converter (i.e., the voltage produced by the AC voltage supply 22 in FIG. 1). One method of controlling the average value of the input voltage is through pulse width modulation of the input voltage. Pulse width modulation of the input voltages is a relatively straightforward method of control that has been widely accepted in the prior art.
Another important characteristic of power converters concerns the nature of their output inductor current. The output inductor current of a power converter may be of a continuous or discontinuous nature. For example, if the magnitude of the output inductor current of the power converter is always positive or always negative but never becomes zero, it is a continuous current. If the magnitude of the output inductor current does become zero between successive positive or negative values, than it is a discontinuous current. A continuous output inductor current is preferable over a discontinuous inductor current in many power converter applications. One advantage of a continuous current is that it is easier to filter than a discontinuous current. Accordingly, a smaller and simpler filter may be used, thus, reducing the size of at least one component in a power converter. The prior art push-pull converter 10, depicted in FIG. 1, produces a continuous output inductor current. As is well known in the prior art, the energy stored in the inductor 19 is discharged, such that a continuous output inductor current is produced.
As can be readily appreciated from the foregoing discussion, there is a need for power converters that have very high power densities. The high density power converters should be capable of being manufactured using high volume production techniques and, thus, inexpensive to produce. Furthermore, the high density power converter should be easily controllable and should, preferably, provide a continuous current output. This invention is directed to a dual transformer device that may be used in power converters to achieve these results.